1. Field of the Invention
This invention relates in general to a wedge loading mechanism for traction drives, and, more particularly, to a wedge loading mechanism having an increased normal force for the contact points of a loading roller wedged between two raceways that form a convergent wedge.
2. Description of Related Art
Traction drives use frictional force to transmit torque and power. Because the power is transmitted between two smooth surfaces, often through a thin layer of lubricant, a traction drive possesses unique characteristics that are not readily attainable by gear drives. These characteristics include quietness, high-efficiency, high rotational accuracy, and zero-backlash.
Generating adequate normal force at the contact is essential for traction drives. Various loading mechanisms have been proposed. These mechanisms have lead to a host of designs. Perhaps the simplest means to generate torque responsive load is using eccentric planetary drives as was disclosed by Dieterich U.S. Pat. No. 1,093,922 in 1914. Over the years, various improvements have been proposed. See for example, U.S. Pat. Nos. 3,945,270, 4,481,842, 4,555,963, and foreign patent numbers JP10-311398, EP 0,856,462 A2. However, these devices sometimes tend to overload rollers and raceways when the wedge angle is small resulting in inefficient transmission of torque and power.
A common practice is to use tapered surfaces along the axial direction. By moving these surfaces axially, a radial displacement and thus normal force are generated. Examples of such designs are disclosed in U.S. Pat. Nos. 3,475,993 and 3,375,739.
Since the envelopes of the tapered surfaces in most designs do not necessarily converge to a common point, this results in a so-called spin motion at contacting surfaces. The spin motion not only offsets the high-efficiency otherwise provided by the traction drive, but also causes component wear and high break away torque.
Recently, a design of zero-spin planetary traction drive has been proposed by Ai as disclosed in the U.S. Pat. No. 6,095,940. This design employs the on-apex concept similar to that of tapered roller bearings. Two rows of planetary rollers are used to balance the internal axial force on the planetary rollers. This design offers torque actuated loading mechanism and greater torque capability.
The cylindrical planetary traction drive is also able to achieve zero-spin motion. However, generating sufficient normal force at the contacts has been a challenge. Designs proposed in the past have offered various means to pre-load the drive either by mechanically deforming the outer rings or by thermal assembling the drive. The pre-load generated by such means, in general, can not be adjusted during operation. For partial load application, traction drives are unnecessarily overloaded. This has negative impacts on transmission efficiency and service life.
As mentioned above, wedge loading provides a simple means to generate normal force. Most of the wedge loading arrangements in prior art were based on a specified friction coefficient, requiring a certain wedge angle corresponding to the friction coefficient. This imposes a restriction to the traction drive design.
Therefore, it is desirable to provide a simple loading mechanism that relieves the restriction on wedge angles, thereby giving improved flexibility for traction drive design.